Pressure sensitive adhesive (PSA) sheets, films and tapes are often difficult to apply to a substrate in a precise location and without air entrapment, primarily due to preadhesion or "quick stick" behavior of the PSA. This is particularly true for "aggressive" PSAs that have high bond strengths and/or low temperature tack properties. Several methods and configurations have been developed to make application of PSA films and tapes easier.
For example, application aids such as detergent and water are commonly used to eliminate preadhesion. Formulations vary, but typically include water, a surfactant or lubricant, and a solvent (generally an alcohol) that speeds bond formation and drying. The liquids tend to form a film between the adhesive and substrate, thus preventing contact and preadhesion. Unfortunately, the liquid is difficult to remove and removal of the liquid is seldom complete. Additionally, most application aids affect the adhesive properties and prevent rapid formation of a strong bond between adhesive and substrate. The application aids may also mar or stain substrate surfaces.
Dry application aids, such as particles, powders or talcs are also useful in preventing preadhesion (See, e.g., U.S. Pat. Nos. 4,376,151 and 4,556,595). While these techniques make application very easy and provide for easy positioning and repositioning, formation of a strong adhesive bond to the substrate is slow, typically requiring contact of 1 to 7 days.
Discontinuous coatings of nontacky materials projecting from the adhesive surface have also been used to prevent preadhesion of the PSA during application. Although these articles can be placed against a substrate and freely moved over the substrate surface, the articles lack any means for weakly and temporarily bonding the adhesive to the substrate prior to permanently bonding the article to the substrate. These adhesive films and tapes have no adhesion when placed lightly against the substrate surface. Such coatings include patterned polymer coatings, particles, and films (See, e.g., U.S. Pat. Nos. 3,554,835; 4,023,570; 4,054,697; 4,151,319; and U.K. Patent No. 1,541,311).
CONTROLTAC brand adhesives use a random distribution of hollow glass microspheres partially embedded in the adhesive as a means to prevent contact and preadhesion (See U.S. Pat. No. 3,331,279). Alternatively, U.S. Pat. No. 3,314,838 describes a similar configuration in which the tops of the hollow microspheres protruding from the adhesive surface are coated by a thin layer of PSA. Pressure application crushes the microspheres permitting the adhesive to contact the substrate and form a strong bond instantly.
Other methods of dealing with preadhesion include formation of brittle plastic projections on a PSA surface (See U.S. Pat. No. 3,301,741), or small discrete clumps of particles protruding from the adhesive surface (See EPO 0 429 269 A1). In the last example, the particles could be tacky adhesive microspheres, thus providing weak bonding to a substrate when applied using light pressure and strong bonding when applied using higher pressures. However, this multi-adhesive construction is generally more difficult to prepare.
Microsphere and patterned adhesives have also been reported that exhibit repositionable properties (See U.S. Pat. Nos. 3,691,140 and 4,166,152). Deformable microspheres, such as described in U.S. Pat. No. 4,735,837 impart a rough or pebble-like surface and repositionable bond. In all these cases, the adhesive films readily form weak, repositionable bonds to a substrate, but do not form a strong, permanent bond.
Topologically structured adhesives have also been described. For example, relatively large scale embossing of an adhesive has been described to permanently reduce the PSA/substrate contact area and hence the bonding strength of the PSA (See EPO 0 279 579). Various adhesive layer topologies include concave and convex v-grooves or hemispheres, and other three dimensional shapes. In general, these topologies provide adhesive sheets, films and tapes with lower peel adhesion values in comparison with smooth surfaced adhesives. In many cases, the topologically structured adhesives also display a change in adhesion with increasing contact time.
The adhesive articles described herein above exhibit various types of positionable and repositionable behavior. Particles added to or applied over an adhesive to change its surface properties tend to alter the bulk adhesive properties. The articles generally suffer from problems of poor release liner attachment, poor aging, handling and bonding characteristics. In sign graphic applications, they tend to be difficult to print without destroying the positionable behavior of the construction. Furthermore, when adhesive articles are prepared using clear films and applied to clear substrates, particles (when pressed against the substrate) are large enough to be seen and result in visual imperfection, particularly, in backlit sign graphic applications.